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Early test results positive at Howard Hanson Dam

The reservoir at Howard Hanson Dam will reach elevation 1,157 feet above sea level within the next 24 hours.

Since April 21 when the reservoir reached the benchmark of 1,147 feet, the U.S. Army Corps of Engineers has conducted intensive monitoring, including scientists on the ground around the clock. Since early March, the Seattle District of the Corps has been slowly filling Howard Hanson Dam’s reservoir for its annual conservation pool.

During the conservation pool, the Corps has been testing and conducting in-depth investigations of the dam’s current effectiveness. The current plan targets a maximum pool elevation of approximately 1,167 feet, unless engineers, geologists and other scientific experts on the ground see anything troubling, then at such time, the reservoir level can be reduced as appropriate.

Initial testing results indicate that water levels at some locations between the grouted seepage barrier and the drainage tunnel are lower than last year, suggesting that the grouted seepage barrier is reducing flow through the north abutment. This data is preliminary, however, and should be considered provisional, as it has not been fully analyzed.

“We can only be encouraged that our data are within expected criteria for a fully functioning seepage barrier,” said Mamie Brouwer, program manager for the dam.

Brower added that tunnel drain improvements, including new horizontal drains, indicate that the efficiency of the tunnel in removing water from the abutment has increased as expected.

The Corps operates the Howard Hanson Dam project, which provides flood damage reduction to the Green River valley, as well as low-flow augmentation to the Green River for environmental benefits. More details and history about Howard Hanson Dam and the right abutment issue can be found at www.nws.usace.army.mil.

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